1. Field of the Invention
This invention relates generally to the field of marine seismic surveying. More particularly, the invention relates to cables and to systems for towing cables through a body of water. The invention is also related to methods and apparatus for reducing drag of cables moving relative to water.
2. Background Art
Marine seismic surveying is known in the art for determining structures and compositions of geologic formations underlying large geographic areas below the bottom of a body of water. One or more seismic vessels tow cables through the water to transport acoustic energy sources and acoustic signal detectors. It is known in the art for a seismic vessel to simultaneously tow a plurality of laterally spaced apart sensor cables in a wide swath through the water. Each cable in the swath may extend thousands of meters behind the seismic tow vessel.
Lateral separation of the sensor cables from each other in a swath is typically performed by marine diverters, a well known type of which is a paravane. The diverters move the cables laterally from the path of vessel travel. Lateral separation of the sensor cables requires that some of the cables in the acquisition system extend laterally from the direction of vessel travel to be pulled through the water. The resistance resulting from pulling cables laterally through the water greatly increases the energy required for towing the cables, resulting in an increase in fuel cost. The turbulence created by pulling cables laterally through the water also increases acoustic noise, and thus results in an increased noise level in the signals detected by seismic (acoustic) sensors deployed in the cables, thereby reducing the signal-to-noise ratio and degrading the quality of the recorded signals.
Cable drag is a significant force that must be overcome by a seismic tow vessel. Cable drag results from skin friction and surface pressure variations induced by viscous effects, and especially those due to separation bubbles or regions. Separation regions result when three-dimensional boundary layers depart from a body surface. Such separation causes vortex shedding which increases cable drag and induces a type of acoustic noise called “strumming.” Such noise interferes with seismic data acquisition and must be filtered from the acoustic signals recorded.
It is not uncommon for seismic energy sources, associated cables and other equipment to produce more than 20,000 pounds (9080 kgf) of drag against a tow vessel Accordingly, a significant amount of energy is expended by a seismic vessel in towing conventional seismic acquisition systems through the water. As the size of marine seismic acquisition systems increases and as the tow speeds increase, a need exists for improved systems to reduce drag forces and to increase tow efficiency through the water.
Compressed air provided by vessel-mounted compressors is used to power air guns in conventional marine seismic operations. In additional to using compressed air to generate seismic signals, it is known in the art to release air bubbles in water to modify the seismic signal generated by marine air guns. For example, U.S. Pat. No. 4,618,024 issued to Domenico (1986), U.S. Pat. No. 4,625,302 issued to Clark (1986), and U.S. Pat. No. 4,632,213 issued to Domenico (1986) each discloses a seismic source system releasing bubbles to form a gas-water mixture paraboloid or “acoustic lens” in the water. Moreover, U.S. Pat. No. 5,959,938 issued to Behrens (1999) discloses a manifold for releasing bubbles in water to control the bubble size and attenuation characteristics of the water. None of these references, however, discloses using gas or air to reduce drag of the seismic array components being towed through the water.
There is a need to increase tow efficiency, a result of which would be fuel savings. Less cable drag could also increase the tow capacity of a seismic vessel and could also enable laterally wider arrays to be towed during seismic surveying.